1. Field of the Invention
The present invention relates generally to testing vacuum systems for a qualifying vacuum. More specifically, the present invention relates to a procedure for performing a bakeout test on a vacuum chamber, such as the chambers used to manufacture integrated circuits on silicon wafers, so that the ion gauge used for measuring the pressure in the vacuum system at a high vacuum does not fail during the test.
2. Background of the Related Art
Vacuum systems are generally known. The manufacture of integrated circuits requires that many manufacturing procedures be performed within a vacuum system. A physical vapor deposition (PVD) chamber is one vacuum system used in the manufacturing of integrated circuits. The vacuum level that a PVD chamber must be able to attain is on the order of 6.0.times.10.sup.-9 torr.
PVD chambers are subjected to a vacuum test to determine the qualifying vacuum pressure for the chamber after the chamber is constructed. Several problems could prevent a chamber from reaching its qualifying pressure, including a leak in the chamber or pump, a leak in the bakeout lamps, or a slow pump. On the other hand, it is possible for a chamber to reach its qualifying pressure, but not show it because of a failed pressure gauge. A typical pressure gauge used at the low pressures of a PVD vacuum system is a Bayard-Alpert ionization gauge (ion gauge), but other types of gauges may be used.
A typical vacuum test, or bakeout procedure, begins with a fully assembled system including a chamber, bakeout lamps, wafer heater, pump section and the measuring devices used to monitor the system's performance during and after the test. The pump section may have more than one pump, including a roughing pump and a high vacuum pump such as a cryogenic pump, a getter pump or a turbomolecular pump.
Initially, the roughing pump is used to pump the system down for about five to ten minutes from atmospheric pressure to about 100 mtorr, close to the limit of a typical roughing pump. Then the pumping action is switched to the high vacuum pump. The high vacuum pump cannot operate at pressures much higher than 100 mtorr, thus the need to use a roughing pump to do the initial pump-down. The high vacuum pump may initially pump the system down for about ten to fifteen minutes to a test pressure of about 3.0.times.10.sup.-6 torr, and eventually achieve the qualifying pressure at about 6.0.times.10.sup.-9 torr after the bakeout. The ion gauge must be turned on at this time to measure the high vacuum in the chamber.
The ion gauge measures the pressure when the high vacuum pump is on. In fact, the ion gauge may not operate properly at the higher pressures prior to switching the pumping action to the high vacuum pump. Therefore, it has become standard practice to turn the ion gauge on when the high vacuum pump is turned on and leave it on during the entire part of the bakeout procedure when the high vacuum pump is on.
The high vacuum pump may hold the high vacuum pressure or ultra high vacuum pressure for the duration of the test. During the test, the bakeout lamps may be turned on to help the chamber degas for about 32 hours. The central wafer heater is initially left off, so it won't suffer too much oxidation. After the initial degassing time with just the bakeout lamps, the wafer heater may also be turned on for about four hours to confirm that it operates and to complete the degassing of the chamber and of itself. After the required time for the bakeout, the bakeout lamps and the wafer heater may be turned off so the chamber can cool down for about twelve to fifteen hours.
After the chamber has cooled down, the pressure is read from the ion gauge to determine if the chamber has reached its qualifying pressure. If the ion gauge has become contaminated in the bakeout process, then it will not show the correct pressure. Instead, it will most likely show a much higher pressure than exists in the chamber, giving the indication that the chamber may have failed the vacuum test. A key indicator that the ion gauge may have failed is that the chamber pressure did not reach its qualifying pressure, but the rate of rise of the pressure is good and no leak can be found in the pump section. Experience has shown that chamber failures due to contaminated ion gauges have been as common, and frequently more common, than failures due to leaks in the chambers.
If a gauge fails, then it has to be replaced, which increases cost of ownership of the vacuum system. Also, the chamber may have to be re-tested, another added cost. Furthermore, the time required to troubleshoot the chamber to determine that it was the gauge that caused the failure, further delays operation of the vacuum system and increases costs.
It is, therefore, desirable to have a method of performing a vacuum system bakeout test that more reliably tests a chamber and avoids failure of the ion gauge.